Perfect, relaxing float!
We took a $30 raft and inner tubes for the 4th of July this year and last. We took one car this year, floated to River Ranch, then walked back on the riverside bike path. There's plenty of parallel parking along the river for free and there's also a bus for $1.75 that runs every hour from River Ranch back up to Tahoe City, but we thought the walk was nicer.
Last year we had two cars, which was more convenient, but the walk was nice. There's a few logs and rock to avoid, so if you're inner tubing a kayak oar isn't a bad idea. It's mostly super slow, lazy and relaxing. Alcohol is banned from the riverbanks and river, but I don't know how well it's enforced. I did see one cop near Tahoe City on the river bank checking rafters out though actually.
The last 40 - 50 feet of the run from Tahoe City to River Ranch is very rocky and swift, so I really recommend getting out just before the wood cabin on the highway side of the river. The bike path goes up a hill just before this stretch and has a wood railing along it.
Great trip, so much fun, and the Lake Tahoe fireworks are awesome!